Move constructor.

Requires: that value_type, error_type and exception_type all implement a move constructor.

Complexity: Same as for value_type’s, error_type’s and exception_type’s move constructors. Constexpr, triviality and noexcept of underlying operations is propagated.

Guarantees: If an exception is thrown during the operation, the object is left in a partially completed state, as per the normal rules for the same operation on a struct.